The ceramic industry has long known and used spinels, e.g., magnesium aluminate (MgAl.sub.2 O.sub.3), both naturally occurring as well as the synthetic spinels, because these materials have high temperature stability. Most of the synthetic spinels have been prepared via the fusion route from mixtures of their respective oxides. These materials as well as the naturally occurring spinels must be ground and formed with the aid of binders since they have little or no reactivity, ability to bond to themselves through crystal reorientation and/or densification, remaining. Therefore, it is necessary to mix the crushed ground or powdered spinel with binders, most of which are changed in chemical structure when the mixture of the spinel and binder is heated to form the final product. In addition, most natural and synthetic spinels contain one or the other of the metals in slight excess, forming a segregated phase among the spinel crystals. The presence of such phases weakens the spinel through disruption of the ordered crystalline structure.
Recently, several patents have issued which provide a means to produce, at low temperatures, precursor compositions which are convertible to their respective spinels in nearly the theoretical ratio of metals, one to the other, which process provides a spinel structure having a considerable amount of reactivity and ability to densify on heating. The unique characteristics of the precursors and their respective low temperature converted oxide forms permit the formation of many products without the aid of binders, and even when a binder is used, the reactivity of the oxide form heated to no higher than about 1200.degree. to 1500.degree. C. prior to final thermal processing, e.g., above 1500.degree. C., produces superior products. The technique described in these patents is to co-precipitate the water soluble salts of the desired metals to form a precursor which is a layered structure of a 1 to 1 ratio of the metal hydroxides, oxyhydroxides, carbonates, nitrates or mixtures thereof with sufficient segregated hydroxide or oxyhydroxide or mixtures of the compounds of the major metal to provide the theoretical spinel ratio of the metals one to the other, e.g., MgAl.sub.2 O.sub. 4. However, most of the techniques taught for such precursor formation have chloride in some small amounts which has been found to create a corrosion problem in some applications.
It would therefore be advantageous to have a process for the preparation of the precursor which avoids the inclusion of, or the use of, any chlorine containing ingredient, including water containing any chlorides.